The present invention relates to an auxiliary steering system for automatically steering a wheeled vehicle so as to improve a cornering characteristic, and a directional stability of the vehicle, in addition to a main steering action commanded by a driver.
A turning behavior and steering stability of a wheeled vehicle are usually described by a relationship between a steering input, and a yaw rate or a lateral acceleration of the vehicle. The steering input is a driver's command to steer the vehicle. In general, a steering angle .theta. of a steering wheel is regarded as the steering input. The yaw rate (or yaw angular velocity) is an angular velocity of a rotation of the vehicle about a vertical axis passing through the center of gravity of the vehicle. The lateral acceleration is an acceleration of the center of gravity of the vehicle in a lateral direction of the vehicle.
It is desired that a vehicle should be turned to an amount corresponding to the driver's steering input without being affected by a disturbance such as a side wind and a friction coefficient of a road surface, and without delay. The amount of a turn of a vehicle can be described in terms of a ratio (yaw rate gain) of the yaw rate .phi. (or the lateral acceleration .alpha.) to the steering input (the steering angle .theta. of the steering wheel). The delay can be described by a lag (phase lag) of an output of the yaw rate (or the lateral acceleration) with respect to the steering input.
In a vehicle having no auxiliary steering control, the yaw rate gain is enhanced at a certain steering frequency, as shown by a curve "a" in FIG. 17, so that the behavior of the vehicle responsive to the steering input is increased abruptly at or near this steering frequency. Furthermore, the phase lag increases rapidly as the steering frequency increases as shown by a curve "a'" in FIG. 17. Therefore, the behavior of the vehicle is retarded with respect to the steering input, so that much skill is required for controlling the vehicle. It is desired that both of the yaw rate gain and the phase lag remain constant over the full range of the steering frequency.
Therefore, there has been proposed an auxiliary steering control system, as shown in FIG. 1, which steers the front or rear wheels in proportion to the yaw rate or lateral acceleration of the vehicle in such a negative direction as to reduce the direction change of the vehicle while the front wheels are steered by the steering input in a conventional manner. However, the auxiliary steering control system of this type is still unsatisfactory in that this system cannot fully eliminate the influence of the disturbance without decreasing the yaw rate gain excessively.